1 62 The Afnerica?i Geologist. September, isou 
difference of the most far-reaching importance exists. The na- 
tional survey here recognizes a great unconformity which is 
taken to represent an erosion interval during the whole of 
Middle and Lower Cambrian time ; the corresponding form- 
ations conseqviently are considered to be absent in this region, 
as well as in many other American localities where the Kee- 
weenawan is typically developed. Thus the Algonkian is made 
to include not only a large part of the Archean of the state sur- 
vey, but also comprises a vast series of rocks (Animikie, Cabot- 
ian, and Manitou) which are ranged by the latter in the Paleo- 
zoic. On the other hand no unconformity at all is admitted 
directly above the Manitou by the state survey, and the Kee- 
weenawan and Animikie are held to represent the Middle and 
Lower Cambrian, though the parallelisms can not be drawn 
more closely. When the fossiliferous members of the Upper 
Cambrian are reached the differences no longer exist. Below 
these beds only three or four undoubted fossils have been 
found (one has been announced as early as the Animikie) and 
these have not proved suf^ciently characteristic to settle any 
of the points in question. There can be no doubt that life ex- 
isted during the Keeweenawan and even during the Huronian, 
not only from theoretical considerations, and from the oc- 
casional presence of abundant carbon, and hydrocarbons, but 
also from the presence of very rare fossils. But until a dis- 
tinctive fauna is discovered and studied no reliance can be 
placed on the pale'ontological evidence. 
According to the latest utterances, therefore the cen- 
tral type rock to be studied is the result of a great refusion 
of Archean greenstones which took place at the close of 
xA.nimikie time. It belongs to the Cabotian series of igneous 
rocks, since the refusion inevitably produced a magma similar 
in all its effects to an ordinary eruptive. This magma con- 
solidating at considerable depth produced gabbros of varying 
composition; consolidating in intruded laccolites and dikes it 
produced diabases; and finally as a surface rock it produced 
amygdaloids, lavas and even probably volcanic ash. When 
the greenstones were more acid from any cause the resultant 
rock may be an augite syenyte or even a granite. 
The following series of rock types were kindly furnished 
